Actually i use investing.com where i have already created portfolio of my trading products and i can easily see the BTC/USD prices over there its ok to use, but for accurate reading you can go with Bitcoinwisdom.com they are best one to use as they provide multiple exchanges prices and also with past 30 days price movement. Mostly there are slippage on every website during some big event when there is some big movement is expected, so you cant always aim for most accurate one.